I think it's important to lowpass sounds in the mix (if you think that sound doesn't really need that bright high end). Because leaving all tops on all sounds as they were might make the top end cluttered and the mix would lack clarity.

IMO, boosting frequencies is fine if that's necessary. Of course that would add color to the sound but it's all good if it works for the mix.

Good skill in EQing comes with a lot of practice. If you're beginner it's hard to distinguish and find those problem frequencies you need to cut. But frequency analysers might help

Types of EQ curvesBell CurveUsed to increase or decrease a wide range of frequencies. (more overall mid range, less overall mid range etc.)PeakUsed to increase or decrease a targeted range of frequencies (More punch in a kick, More bite in a snare).NotchUsed to decrease a specific problem frequency (A resonance in a single bass note or synth, a ring in a snare drum). 449hz -12.8 db Q 8.57High ShelfUsed to decrease or increase the overall brightness (adds air to a vocalist, add sizzle to a synth or snare). 3.54hz 7.56db Q 0.71Low ShelfUsed to increase or decrease the overall bass (reduce rumble in a sound, increase low end power in a bass). 255z -6.32db Q 0.71High Pass FilterUsed to eliminate bass frequencies (clean up unwanted bass from a hi hat or vocalist, sweep for effect).Low Pass FilterUsed to eliminate high frequency (clean up unwanted high frequencies on a bass or synth, sweep for effect) 2.37 kHz 0.00db Q 0.72Band Stop FilterUsed to eliminate a group of frequencies (seep for effect). 790hz 0.00db Q 0.61

To add on to this, you will often see these grouped into larger categories of EQ terms. Shelves and Filters are the more obvious of the two, but most of the time people just group peak, notch, and bell curve altogether under term "band".

Another point is that when you increase the Q value on a Band Stop filter, you will sometimes see this called as a "Notch Filter". The difference between this and a notch band is comparable to that between a high-pass filter and a low shelf.

Also you will sometimes see low-pass filters be called hi-cut or hi-pass filters be called low-cut.

A lot of issues you may stumble upon right away will be sheerly due to the wide variety of terminology, but it isn't really something to worry about. Just stick with whatever your EQ of choice calls it and once time comes along it'll be easier to adapt.

I have a very unorthodox approach to EQ because I have zero formal training so I have a lot to learn, but one of the most important things I do, for me, is putting a hard 4x eq high pass and low pass at the end of every chain on every instrument. This part might might sound dumb but I close my eyes and slowly close the filter until I hear a noticeable change in timbre. Often times I'll remove huge amounts of high and low end from sounds, much more than anticipated.

Timbre, in my opinion, is the most important aspect of a production. I will design a new sound to use with certain notes just to make sure the timbre is right. With EQ, I think abandoning certain "rules" is incredibly important to make sure timber stays in tact. For example, I will use volume modulation with volume shaper by cableguys or any other "brick-wall" style compressor to give instruments room to breathe at certain frequency ranges before I start cutting sacrificing frequencies. In the song in my profile the main bass pluck hits hard in the 100-200 range, and so does the sub that I layered it on, which interferes with my kick. If I cut those frequencies it loses its character, so I brickwall them for almost an eighth note to allow the kick to get its time in the limelight. The sub pluck hits at that frequency very early in the sound and dives by 12 semitones, where the mid bass doesn't hit that area until later in the envelope, so they don't particularly overlap.

I think this is a long winded way of saying that over EQing can turn songs into a brick that is a little too neat and loses its soul. I like to listen to songs that I think have good production (read: doesn't have to be a mord fustang or madeon or whoever tune), and throw them in ableton then listen to certain frequency bands using a filter to see whats going on. Most of the time you will hear all kinds of shit besides just the kick drum or sub bass hitting in the sub 100hz range, which makes you wonder why some people are so adamant about high passing everything.

I'm easily the most annoying poster on here. All my shit is long winded.

TL:DR version:EQ is a matter of taste as long as your songs aren't muddy and don't clip.

This sweeping technique is very useful on drums, specially rides. There can be a lot of ringing/whistling freqs in a ridehat. A good rule of thumb is to cut a lot on the very annoying frequencies and cut less on the less annoying frequencies. As said before though, in some sounds you can find a lot of ringing frequencies and if you take them all out you will ruin the sound. It's important to find a good balance and to A/B during the process.